1. Field of the Invention
The present invention relates to a reducer of an electric power steering apparatus. More particularly, the present invention relates to a reducer of an electric power steering apparatus, which can exactly assist a driver's operation of a steering wheel by minimizing an increase in the torque, caused by moisture absorption of a worm wheel, and a size change due to the moisture absorption, and also can eliminate the playing caused by wearing of a worm and the worm wheel, and inhibit the occurring of a rattle noise caused by a backlash and a reversely input load.
2. Description of the Prior Art
Recent research development of a technology on a steering apparatus provided in a vehicle shows a tendency toward achieving a driver's convenience (such as stability of steering state, and soft and comfortable steering feeling) as well as safety. According to such a development tendency, in order to improve the steering feeling of a driver, various power steering apparatuses such as a hydraulic power steering apparatus or an electric power steering apparatus have been developed. Compared to a hydraulic power steering apparatus, an electric power steering apparatus is light-weight, requires low energy consumption, and improves the fuel efficiency of a vehicle. Thus, due to such advantages, the use of the electric power steering apparatus shows a tendency to be increased.
In general, the electric power steering apparatus assists a steering torque of a driver by driving a motor in an electronic control device according to a vehicle's driving conditions sensed by a wheel speed sensor, a torque sensor, etc. while providing a light and comfortable steering state during low-speed driving, providing a heavy and stable steering state during high-speed driving, and allowing rapid steering operation to be performed in a state of accidental emergency so as to cope with the situation, which provides steering performance allowing the driver's optimum steering feeling to be maintained.
FIG. 1 is a configuration view illustrating a conventional electric power steering apparatus. As shown in FIG. 1, an electric power steering apparatus 100 is configured to include a steering system 130 extending from a steering wheel 102 to both-side wheels 126, and an auxiliary power mechanism 140 for supplying auxiliary steering power to the steering system 130.
The steering system 130 includes a steering shaft 106 whose one side is connected to the steering wheel 102 and rotates together with the steering wheel 102, and the other side is connected to a pinion shaft 108 via a couple of universal joints 104. Also, the pinion shaft 108 is connected to a rack bar 112 through a rack-pinion mechanism 110, and both ends of the rack bar 112 are connected to the wheels 126 of a vehicle through a tie rod 122 and a knuckle arm 124.
The auxiliary power mechanism 140 includes a torque sensor 142, an electronic control unit 144, a motor 146, and a reducer 150. The torque sensor 142 senses a torque applied to the steering wheel 102 by a driver, and outputs an electrical signal in proportion to the sensed torque. The electronic control unit 144 generates a control signal based on the electrical signal transferred from the torque sensor 142. The motor 146 generates auxiliary steering power based on the control signal transferred from the electronic control unit 144. The reducer 150 is provided with a worm 152 and a worm wheel 156 so as to transfer the auxiliary steering power generated by the motor 146 to the steering shaft 106.
Accordingly, the electric power steering apparatus is configured in such a manner that the torque generated by the rotation of the steering wheel 102 is transferred to the rack bar 112 via the rack-pinion mechanism 110, and the auxiliary steering power generated by the motor 146 according to the generated torque is transferred to the rack bar 112.
In other words, the torque generated by the rotation of the steering wheel 102, together with the auxiliary steering power generated by the motor 146, allows the rack bar 112 to move in the axial direction.
FIG. 2 is a cross-sectional view illustrating a reducer of a conventional electric power steering apparatus. As shown in FIG. 2, in the reducer 150, a worm shaft 254 formed with the worm 152 is provided, and both ends of the worm shaft 254 are provided with worm shaft bearings 257, respectively, to support the worm shaft 254. Also, in order to prevent the worm shaft bearing 257 from playing in the axial direction of the worm shaft 254, a plug bolt 210 is fastened between a damping coupler 240 and the worm shaft bearing 257, and the plug bolt 210 is fixed by a plug nut 220.
The worm shaft 254 is configured to be connected to the motor 146 via the damping coupler 240 and to be rotated by the driving of the motor 146.
Then, at one side of the external diameter of the worm 152, the worm wheel 156 is provided in such a manner that it can be tooth-engaged with the worm 152 formed on the worm shaft 254. The worm wheel 156 is configured to be mounted to the steering shaft 106, in which to the steering shaft 106, the torque of the steering wheel 102 operated by the driver is transferred, while the worm shaft 254's auxiliary steering power caused by the driving of the motor 146 is transferred to the steering shaft 106.
A gear housing 260 houses the worm 152, the worm wheel 156, and the like, and is provided with, at its one side, the motor 146 for providing the driving force to the worm shaft 254. The gear housing 260 and the motor 146 are combined with each other through a bolt 250 by a motor cover 230.
The worm shaft bearing 257 has a ball 258 combined with an inner wheel 280 and an outer wheel 270 thereof so as to support the rotation of the worm shaft 254 connected to the motor 150 at the end of the gear housing 260.
The reducer of the electric power steering apparatus, as configured above, controls the driving of a motor by an electronic control unit provided in a vehicle according to driving conditions of the vehicle, and transfers a worm shaft's auxiliary steering power caused by the driving of the motor, together with a driver-driven steering wheel's torque, to a steering shaft. This makes it possible to keep the driver's steering state soft and stable.
However, the reducer of the conventional electric power steering apparatus has a problem in that the progress in the wearing of the worm and the worm wheel rotated by the driving of the motor causes playing due to the wear, and a rattle noise due to the backlash. Furthermore, there is a problem in that the rattle noise more significantly occurs by a load reversely input from wheels during driving on a rough road.
Also, the worm is made of a metal such as steel while a part (outer circumferential portion) or the whole of the worm wheel is made of a plastic material to cope with a friction resistance with the worm. Thus, when a vehicle is driven in a region of high temperature and humidity, the worm wheel made of the plastic material is expanded by absorption of moisture, thereby changing the size of the worm wheel. This increases the torque, making a driver's steering feeling heavy, which reduces the performance.